Bsc Zoology-Complementary notes-Classification of mollusca

Class 1. Cephalopoda

1. The foot is modified into a finuiel and tentacles (aims) present around the mouth. Hence the name Cephalopoda.

2. The body has a bilateral symmetry.

3. The head is prominent and bears two large eyes.

4. The visceral mass is not coiled.

5. The shell is present or absent. It is external or internal.

6. The mouth is provided with jaws and radula.

 7. Gills are used for respiration.

8. The heart has two or four auricles and one ventricle.

9. Two pairs of kidneys are present.

10. Sexes are separate.

 11. Development is direct

12. They are exclusively marine..

Class 2. Pelecypoda (Bivalvia)

I. They have bilateral symmetry.

2. The body is laterally compressed.

3. The shell consists of two valves and hence the name Bivalvia.

4. The foot is wedge shaped and hence the name Pelecypoda.

S. The head is absent

6. Mouth and anus are at the opposite ends.

 7. Mantle is formed of two lobes.

8. Radula is absent.

9. Alimentary canal shows a crystal-line style.

10. Respiration is by paired gills.

11. Kidneys are paired.

12. Sexes are separate.

13. Gonads open into the mantle cavity.

14. Development includes trocho-phore, veliger and glochidium larva.

15. They are aquatic.

16. They are sedentary or burrowing in habit.

Class 3. Gastropoda

 I. Gastro = stomach; poda = foot; It seems that these animals are moving on their stomach. Hence the name gastropoda.

 2. Gastropods are marine, freshwater or terrestrial animals. A few are parasitic.

3. The body is unsegmented and asym-metrical.

4. The shell is univalved and spirally coiled.

5. The head is distinct. It bears ten-tacles, eyes and a mouth.

6. Respiration takes place by gills or lungs or by both.

 7. The visceral mass is turned through 180° by torsion.

8. Mouth and anus lie on the same side.

9. The foot is ventral and muscular.

10. The buccal cavity is provided with a radula.

II. The circulatory system is open.

12. The sexes are mostly separate, while some forms are hermaphrodite.

13. The development includes veliger and trochophore larvae.